JP2005243151A - Diffraction optical element, objective lens, optical pickup device, and optical information recording and reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element which can adequately make corrections such as axial chromatic aberration and the like to luminous flux of short wavelength in spite of a high allowance to a production error, a diffraction optical element and an objective lens used for an optical pickup device having compatibility to optical information recording media different in thickness of protective layer, and an optical pickup device and an optical information recording and reproducing device using the objective lens. <P>SOLUTION: In the diffraction structure of a diffraction optical element, Δ'=(dor'/dor)Δ, wherein De is an effective radius, dor-degree diffraction is a diffraction structure within a prescribed height h from an optical axis X, dor'-degree diffraction is a diffraction structure out of a prescribed height h from the optical axis X, Δ is the width of a diffraction ring zone in case of the dor-degree diffraction, and Δ' is the width of a diffraction ring zone in case of the dor'-degree diffraction. When dor' is larger than dor, Δ' is consequently larger than Δ. Therefore, the width of a diffraction ring zone can be increased in a diffraction structure out of a prescribed height h. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a diffractive optical element suitable for use in an optical pickup device that reproduces and / or records information on an optical information recording medium using a light beam having a short wavelength, and an objective lens including the same. The present invention relates to an optical pickup device and an optical information recording / reproducing device.

  In recent years, DVD (digital versatile disc; abbreviated as DVD) is rapidly spreading as an optical information recording medium for recording video information and the like. A DVD can record information of 4.7 GB per surface by using a red semiconductor laser having a wavelength of 650 nm and an objective lens having a numerical aperture (NA) of 0.6 in an optical pickup device mounted on the player. .

  However, since one DVD can record high-definition image information only for about 30 minutes per side, it is pointed out that the capacity is too small to be used as an optical information recording medium in the coming digital high-definition broadcasting era. . Against this background, in recent years, research and development of a high-density recording optical disk system using a blue-violet semiconductor laser with a wavelength of 405 nm and an objective lens with NA of 0.85 has progressed. (Blu-ray Disc; abbreviated as BD) was announced in February 2002. Since a BD has a recording capacity of about 23.3-27 GB per side, if it is used, high-definition image information can be recorded for about 2 hours per side. There is also a standard for high density optical disks called HD DVDs. In this specification, an optical disk using a blue-violet semiconductor laser as a recording / reproducing light source is generically called a “high density optical disk”.

JP 2003-167190 A

  However, correction of axial chromatic aberration is indispensable for a short wavelength light beam emitted from a blue-violet semiconductor laser. In contrast, Patent Document 1 discloses a diffractive optical element in which a diffractive structure is provided on an optical surface in order to correct axial chromatic aberration. Such a diffractive optical element is advantageous in terms of cost because it can correct such longitudinal chromatic aberration with a single lens configuration. However, in general, when an annular diffractive structure is provided on the optical surface, the width of the diffractive annular zone becomes smaller in proportion to the correction amount of the axial chromatic aberration, so that a relatively large axial chromatic aberration correction is required. In such a short-wavelength diffractive optical element, the diffractive ring zone becomes extremely fine, particularly in the vicinity of the periphery of the effective diameter, and it becomes difficult to form such a fine diffractive ring zone in the mold of the diffractive optical element. There is also a risk of transfer failure during molding of the optical element. If the diffraction ring zone cannot be formed on the optical surface according to the design value, the diffraction efficiency is lowered, so that desired optical characteristics cannot be exhibited. The diffractive optical element described in Patent Document 1 does not disclose any solution to such a problem.

In recent years, for example, in order to ensure compatibility, a single optical system is used to condense light beams from a plurality of light sources having different wavelengths, so that optical disks having different protective layer thicknesses such as BD and DVD can be used. On the other hand, an optical pickup apparatus that appropriately records and / or reproduces information has been developed. In such an optical pickup apparatus, compatibility with optical disks having different operating wavelengths is achieved by using a diffractive optical element. In such a diffractive optical element for an optical pickup device having compatibility, in addition to correcting axial chromatic aberration, it is necessary to correct spherical aberration due to the difference in the thickness of the protective layer, and the width of the diffraction ring zone is further reduced. Therefore, the above problem becomes more obvious.

  The present invention has been made in view of such a problem of the prior art, and is a diffraction that can appropriately correct axial chromatic aberration and the like for a short-wavelength light beam despite a high tolerance for manufacturing errors. It is an object to provide an optical element, a diffractive optical element that can be used in an optical pickup device having compatibility with optical information recording media having different protective layer thicknesses, and an optical pickup device and an optical information recording / reproducing device using the same. .

The diffractive optical element according to claim 1, wherein the diffractive optical element has a diffractive surface on which a diffractive structure including a plurality of annular zone steps is formed.
Depth d _IN in the optical axis direction of the annular zone step inside the predetermined height from the optical axis in the optical surface, and a depth d _{OUT in} the optical axis direction of the annular zone step outside the predetermined height, Each of them satisfies the following expressions (1) to (3).
m = Int (| d _IN | · (N−1) / λ) (1)
n = Int (| d _OUT | · (N−1) / λ) (2)
m <n (3)
Where Int (X) is an integer closest to X, λ is the wavelength of the light beam incident on the diffractive structure, and N is the refractive index of the diffractive optical element at the wavelength λ.

  When the diffractive optical element is designed so that, for example, the diffraction order is doubled, the diffraction angle of the light beam is unchanged, that is, the optical characteristics are unchanged, but the width of the diffraction zone is doubled. It uses characteristics. That is, in the case of a diffractive structure having the same diffraction order within the effective diameter, the width of the diffraction zone generally becomes finer as it goes to the periphery of the effective diameter. May become difficult. On the other hand, by increasing the diffraction order in the middle of the effective diameter, the width of the diffraction ring zone around the effective diameter is increased while maintaining the optical characteristics of the diffractive optical element, and the diffraction structure is formed during actual manufacturing. It is easy.

When the diffraction order in the diffractive structure is do, the step is d, the wavelength is λ, and the refractive index is N, the step d is expressed by the following general formula: d = dor · λ / (N−1) (13)
Therefore, if the diffraction order dor is set to a larger dor 'in the middle of the effective diameter, the depth of the step becomes (dor' / dor) times, but the width of the diffracting zone is also (dor '/ dor). Therefore, it is possible to provide a diffraction structure that is easy to manufacture.

  More specifically, in the schematic cross-sectional view of the diffractive structure of the diffractive optical element shown in FIG. 11, the effective radius is De, the diffractive structure within a predetermined height h from the optical axis X is the dor-order diffraction, and the light If the diffraction structure outside the predetermined height h from the axis X is defined as dor′-order diffraction, and the width of the diffraction ring zone in the case of dor-order diffraction is Δ, the width of the diffraction ring zone in the case of dor′-order diffraction Δ ′ Becomes (dor ′ / dor) Δ. However, if dor ′> dor, Δ ′> Δ, so that the width of the diffraction zone can be increased in the diffractive structure outside the predetermined height h.

  When the diffractive structure is formed on a spherical surface or an aspherical surface, all the steps d within the effective diameter do not exactly match the above equation (13). This is because the step d is determined in consideration of the refractive action in addition to the diffraction action. Generally, the deviation from the above equation (13) increases as the distance from the optical axis increases. For example, when a diffractive structure having a positive diffractive power is formed on an aspheric surface having a positive paraxial power, the step d tends to increase as the distance from the optical axis increases.

  Therefore, in the diffractive optical element according to claim 1, the diffraction order of the diffractive structure inside the predetermined height from the optical axis in the optical surface is defined by the above equation (1), and a predetermined order is determined from the optical axis in the optical surface. The diffraction order of the diffraction structure outside the height was defined by the above equation (2).

  The diffractive optical element according to claim 2 is characterized in that, in the invention according to claim 1, the wavelength λ is 450 nm or less.

  In the blue-violet wavelength region, a relatively large axial chromatic aberration correction is required, so the width of the diffraction zone tends to be narrow. However, in the diffractive optical element according to the present invention, the diffraction order is switched and increased in the middle of the effective diameter. Therefore, it is possible to prevent a decrease in diffraction efficiency due to a shape error.

The diffractive optical element according to claim 3 is characterized in that, in the invention according to claim 1 or 2, the diffraction efficiency is maximized when the light flux having the wavelength λ is incident on the annular zone step inside the predetermined height. The d _IN is set so that the diffraction order at which the diffraction efficiency becomes maximum when the luminous flux having a wavelength longer than the wavelength λ is incident on the annular zone step inside the predetermined height becomes smaller than the diffraction order. It has been determined.

When the diffractive optical element according to the present invention is used in an optical pickup device having compatibility with a plurality of optical disks having different operating wavelengths, a diffractive structure inside a predetermined height is used as described in claim 3. by diffraction order of the diffracted light of the light beam generated wavelength longer than the wavelength λ than the diffraction order of the diffracted light of the light beam having the wavelength λ determines the level difference d _iN to be lower, even for the light flux of any wavelength It becomes possible to ensure high diffraction efficiency.

  The diffractive optical element according to claim 4 is characterized in that, in the invention according to claim 3, the wavelength λ is 450 nm or less, and a wavelength longer than the wavelength λ is in a range of 640 nm to 690 nm.

The configuration described in claim 3 is particularly effective when the two wavelengths are within the range described in claim 4. Specifically, the following combinations of the diffraction order m ₁ of the diffracted light of the light beam having the wavelength λ of 450 nm or less and the diffraction order m ₂ of the diffracted light of the light beam having a wavelength in the range of 640 nm to 690 nm are preferable.
(M ₁ , m ₂ ) = (2,1), (3,2), (4,2), (5,3), (6,4), (7,4), (8,5), (9,6), (10,6)
Of these combinations of diffraction orders, (m ₁ , m ₂ ) = (2, 1), (3, 2), (5, 3) are particularly preferable.

The diffractive optical element according to claim 5 is the refracting optical element according to claim 4, wherein the wavelength λ is λ ₁ , the wavelength longer than the wavelength λ is λ ₂ , and the refractive index of the diffractive optical element is the wavelength λ _1. N _λ1 , the refractive index of the diffractive optical element at the wavelength λ ₂ is N _λ2 , and the light flux having the wavelength λ ₁ is incident on the annular step inside the predetermined height, the diffraction efficiency is maximum. When the diffraction order is m ₁ , and the diffraction order at which the diffraction efficiency becomes maximum when the light flux having the wavelength λ ₂ is incident on the annular zone step inside the predetermined height is m ₂ , the following (4 The diffraction structure is characterized in that it has a wavelength dependency of the spherical aberration such that the spherical aberration changes in the overcorrection direction when the wavelength of the incident light beam becomes long.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)}> 1 (4)

According to a fifth aspect of the present invention, there is provided an optical pickup device having compatibility with a plurality of optical discs having different working wavelengths and protective layer thicknesses using the diffractive optical element according to the present invention. This is a preferred configuration when used as a compatible diffractive optical element for correction. When the refractive index and diffraction order for each wavelength satisfy the above equation (4), the wavelength dependence of the spherical aberration such that the spherical aberration changes in the overcorrected direction when the wavelength of the incident light beam in the diffractive structure becomes longer. Thus, it is possible to correct spherical aberration due to the difference in the protective layer thickness of a plurality of optical disks. In the diffractive optical element having such a configuration, a preferable combination of diffraction orders is (m ₁ , m ₂ ) = (2, 1).

A diffractive optical element according to a sixth aspect is the refracting optical element according to the fourth aspect, wherein the wavelength λ is λ ₁ , the wavelength longer than the wavelength λ is λ ₂ , and the refractive index of the diffractive optical element is the wavelength λ _1. N _λ1 , the refractive index of the diffractive optical element at the wavelength λ ₂ is N _λ2 , and the light flux having the wavelength λ ₁ is incident on the annular step inside the predetermined height, the diffraction efficiency is maximum. When the diffraction order is m ₁ , and the diffraction order at which the diffraction efficiency is maximum when the luminous flux of wavelength λ ₂ is incident on the annular zone step inside the predetermined height is m ₂ , the following (5 The diffraction structure is characterized in that it has a wavelength dependency of the spherical aberration so that the spherical aberration changes in the direction of insufficient correction when the wavelength of the incident light beam becomes longer.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)} <1 (5)

The invention according to claim 6 is also an optical pickup device having compatibility with a plurality of optical discs having different working wavelengths and protective layer thicknesses, as in the invention according to claim 5. This is a preferred configuration when used as a compatible diffractive optical element for correcting spherical aberration due to a difference in thickness of the protective layer. When the refractive index and diffraction order for each wavelength satisfy the above equation (5), the wavelength dependence of the spherical aberration such that the spherical aberration changes in the direction of insufficient correction when the wavelength of the incident light beam in the diffractive structure becomes longer. Thus, it is possible to correct spherical aberration due to the difference in the protective layer thickness of a plurality of optical disks. In the diffractive optical element having such a configuration, a preferable combination of diffraction orders is (m ₁ , m ₂ ) = (3, 2).

  The diffractive optical element according to claim 7 is the diffractive optical element according to any one of claims 1 to 6, wherein the diffractive optical element further includes an optical surface having a negative refractive power in a paraxial axis, and the diffractive structure. Is formed on an optical surface whose refractive power in the paraxial axis is negative, and the cross-sectional shape including the optical axis of the diffractive structure is a staircase shape. Typical sectional views of the diffractive structure having such a configuration are shown in FIGS. 4, 6, and 8.

  The diffractive optical element according to an eighth aspect of the present invention is the diffractive optical element according to any one of the first to sixth aspects, wherein the diffractive optical element further includes an optical surface whose refractive power in the paraxial axis is negative, and the diffractive structure. Is formed on a macroscopic plane opposite to the optical surface where the refractive power in the paraxial is negative, and the cross-sectional shape including the optical axis of the diffractive structure is a sawtooth shape. FIG. 10 shows a schematic cross-sectional view of the diffractive structure having such a configuration.

A diffractive optical element according to a ninth aspect is the invention according to any one of the first to eighth aspects, wherein the diffraction power in the paraxial axis of the diffractive structure with respect to the wavelength λ is P ₁ (mm ⁻¹ ), and the wavelength λ. The diffraction power at the paraxial axis of the diffraction structure for a wavelength longer than 5 nm is P ₂ (mm ⁻¹ ), and the diffraction power at the paraxial axis of the diffraction structure for a wavelength 5 nm shorter than the wavelength λ is P ₃ (mm ⁻¹ ). At this time, since the following expression (6) is satisfied, axial chromatic aberration can be effectively corrected.
P ₂ > P ₁ > P ₃ (6)

An objective lens according to a tenth aspect is characterized by being integrated with the diffractive optical element according to any one of the first to ninth aspects.

  When the diffractive optical element according to any one of claims 1 to 9 is integrated with an objective lens and tracking driving is performed, coma aberration due to decentering of the diffractive optical element and the objective lens does not occur, so that good tracking is achieved. It becomes possible to obtain characteristics.

The optical pickup device according to claim 11, wherein the optical pickup device includes a first light source that emits a light beam having a _first wavelength λ ₁ and a condensing optical system including an objective lens. An optical pickup capable of recording and / or reproducing information by condensing the light beam from the first light source on the information recording surface of the first optical information recording medium via the system In the device
The condensing optical system has a diffractive optical element having a diffractive surface on which a diffractive structure composed of a plurality of annular zone steps is formed,
In the diffractive optical element, the inside of the annular zone step from a predetermined height from the optical axis of the optical surface and the depth d _IN of the optical axis, the optical axis direction of the annular zone step outside than the predetermined height The depth d _OUT satisfies the following expressions (7) to (9), respectively. The operational effects of the present invention are the same as those of the first aspect of the present invention.
m = Int (| d _IN | · (N _λ1 −1) / λ ₁ ) (7)
n = Int (| d _OUT | · (N _λ1 −1) / λ ₁ ) (8)
m <n (9)
Where Int (X) is an integer closest to X, λ ₁ is the wavelength of the light beam incident on the diffractive structure, and N _λ1 is the refractive index of the diffractive optical element at the wavelength λ _1. is there.

An optical pickup device according to a twelfth aspect is characterized in that, in the invention according to the eleventh aspect, the first wavelength λ ₁ is 450 nm or less. The effect of the present invention is the same as that of the invention described in claim 2.

The optical pickup device according to a thirteenth aspect is the invention according to the eleventh or twelfth aspect, wherein the optical pickup device further includes a second wavelength λ ₂ (λ ₂ > λ that is longer than the first wavelength λ _{1. 1} ) by having a second light source that emits the luminous flux of ₁ ) and condensing the luminous flux from the second light source on the information recording surface of the second optical information recording medium via the condensing optical system. , It is possible to record and / or reproduce information,
For the diffraction order at which the diffraction efficiency is maximized when the light beam of the first wavelength λ ₁ is incident on the annular zone step inside the predetermined height, the light beam of the second wavelength λ ₂ is The _dIN is determined so that the diffraction order that maximizes the diffraction efficiency when entering the annular zone step inside the predetermined height is reduced. The function and effect of the present invention are the same as those of the third aspect of the present invention.

The optical pickup device according to a fourteenth aspect is the optical pickup device according to the thirteenth aspect, wherein the first wavelength λ ₁ is 450 nm or less and the second wavelength λ ₂ is in the range of 640 nm to 690 nm. It is characterized by. The function and effect of the present invention are the same as those of the invention described in claim 4.

An optical pickup device according to a fifteenth aspect is the optical pickup apparatus according to the fourteenth aspect, wherein the refractive index of the diffractive optical element at the _first wavelength λ ₁ is N _λ1 , and the second wavelength λ ₂ is the refractive index. The refractive index of the diffractive optical element is N _λ2 , the diffraction order that maximizes the diffraction efficiency when the light beam having the first wavelength λ ₁ is incident on the annular zone step inside the predetermined height is m ₁ , When the light beam having the second wavelength λ ₂ is incident on the annular step inside the predetermined height and the diffraction order that maximizes the diffraction efficiency is m ₂ , the following expression (10) is satisfied. The diffraction structure is characterized in that the spherical aberration has a wavelength dependency such that the spherical aberration changes in the overcorrection direction when the wavelength of the incident light beam becomes long.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)}> 1 (10)
The function and effect of the present invention are the same as those of the fifth aspect of the invention.

An optical pickup device according to a sixteenth aspect is the optical pickup device according to the fourteenth aspect, wherein the refractive index of the diffractive optical element at the _first wavelength λ ₁ is N _λ1 , and the second wavelength λ ₂ is the refractive index. The refractive index of the diffractive optical element is N _λ2 , the diffraction order that maximizes the diffraction efficiency when the light beam having the first wavelength λ ₁ is incident on the annular zone step inside the predetermined height is m ₁ , When the light beam having the second wavelength λ ₂ is incident on the annular zone step inside the predetermined height and the diffraction order that maximizes the diffraction efficiency is m ₂ , the following expression (11) is satisfied. The diffractive structure is characterized in that the spherical aberration has a wavelength dependency such that the spherical aberration changes in the direction of insufficient correction when the wavelength of the incident light beam becomes long.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)}> 1 (11)
The effect of the present invention is the same as that of the invention described in claim 6.

  An optical pickup device according to a seventeenth aspect is the invention according to any one of the eleventh to sixteenth aspects, wherein the diffractive optical element further includes an optical surface whose refractive power in the paraxial axis is negative, and the diffractive structure. Is formed on an optical surface whose refractive power in the paraxial axis is negative, and the cross-sectional shape including the optical axis of the diffractive structure is a staircase shape. The function and effect of the present invention are the same as those of the seventh aspect of the invention.

  An optical pickup device according to an eighteenth aspect is the optical pickup device according to any one of the eleventh to sixteenth aspects, wherein the diffractive optical element further includes an optical surface whose refractive power in a paraxial axis is negative, and the diffractive structure. Is formed on a macroscopic plane opposite to the optical surface where the refractive power in the paraxial is negative, and the cross-sectional shape including the optical axis of the diffractive structure is a sawtooth shape. The function and effect of the present invention are the same as those of the eighth aspect of the invention.

An optical pickup device according to a nineteenth aspect is the invention according to any one of the eleventh to eighteenth aspects, wherein the diffraction power in the paraxial axis of the diffractive structure with respect to the first wavelength λ ₁ is P ₁ (mm ⁻¹ ). P ₂ (mm ⁻¹ ) is the paraxial power of the diffractive structure for the wavelength 5 nm longer than the first wavelength λ _{1, and} the paraxial power of the diffractive structure is 5 nm shorter than the first wavelength λ ₁ . When the diffraction power is P ₃ (mm ⁻¹ ), the following expression (12) is satisfied.
P ₂ > P ₁ > P ₃ (12)
The function and effect of the present invention are the same as those of the ninth aspect of the invention.

  An optical pickup device according to a twentieth aspect is characterized in that, in the invention according to any one of the eleventh to nineteenth aspects, the diffractive optical element is integrated with the objective lens. The effect of the present invention is the same as that of the invention described in claim 10.

  An optical information recording / reproducing apparatus according to claim 21 is an optical pickup apparatus according to any one of claims 11 to 20, and the optical information can be recorded and / or reproduced by the optical pickup apparatus. And an optical information recording medium supporting means for supporting the recording medium.

  In the present specification, the “objective lens” is arranged on a position opposite to the optical disk in the optical pickup device, and light beams emitted from the light sources having different wavelengths are recorded on the information recording surfaces of the optical disks having different recording densities. It points out the condensing element which has the function to condense. In addition, when there is an optical element that can be operated at least in the optical axis direction by an actuator together with the condensing element, an optical system constituted by the condensing element and the optical element is used as an objective lens.

  The diffractive structure used in this specification refers to a form in which a relief is provided on the surface of the optical element to give a diffractive action. When there is a region where diffraction occurs on one optical surface and a region where it does not occur, A region where diffraction occurs. As the shape of the relief, for example, it is formed on the surface of the optical element as a substantially concentric annular zone centered on the optical axis, and each annular zone is like a staircase or a sawtooth if the cross section is viewed in a plane including the optical axis. Shapes are known, but include such shapes.

  In the present specification, the first optical information recording medium refers to a high-density optical disk that uses a blue-violet semiconductor laser as a recording / reproducing light source, such as BD (Blu-ray Disc) or HD DVD. The optical information recording medium is a DVD-ROM, DVD-Video, DVD-Audio used exclusively for reproduction, various DVD-type optical disks such as DVD-RAM, DVD ± R, DVD ± RW, etc. A CD-type optical disk such as a CD-ROM, CD-R, CD-RW and the like.

  According to the present invention, a diffractive optical element capable of appropriately correcting axial chromatic aberration and the like with respect to a light beam having a short wavelength despite an high tolerance for manufacturing errors, and an optical information recording medium having different protective layer thicknesses It is possible to provide a diffractive optical element that can be used in an optical pickup device having compatibility with the above, an objective lens, an optical pickup device using the same, and an optical information recording / reproducing device.

Embodiments of the present invention will be described below with reference to the drawings.
<First Embodiment>
FIG. 1A shows a first light that can appropriately record / reproduce information on both the high-density optical disk HD that is the first optical information recording medium and the optical disk DVD that is the second optical information recording medium. It is a figure which shows schematically the structure of pick-up apparatus PU1. FIG. 1B is a front view of a light source used in the optical pickup device of FIG. The optical pickup device PU1 and a support means (not shown) for supporting the optical disk constitute an optical information recording / reproducing device.

  The optical specifications of the high-density optical disk HD are as follows: (first) wavelength λ1 = 405 nm of the light beam used for recording and / or reproducing information, the thickness t1 of the first protective layer PL1 = 0.1 mm, and the first numerical aperture NA1. = 0.85, and the optical specifications of the optical disc DVD are (second) wavelength λ2 = 655 nm of the light beam used for recording and / or reproducing information, the thickness t2 of the second protective layer PL2 is 0.6 mm, The second numerical aperture NA2 = 0.65. However, the combination of the wavelength, the thickness of the protective layer, and the numerical aperture is not limited to this.

  The optical pickup device PU1 emits light when recording / reproducing information with respect to the high-density optical disc HD, and emits a laser beam of 405 nm and emits a 405 nm laser beam and the optical disc DVD. The second light emitting part (second light source) EP2 that emits light when emitting / recording information and emits a 655 nm laser light beam and the reflected light beam from the information recording surface RL1 of the high-density optical disk HD are received. A high-density optical disk / DVD laser module LM composed of a first light-receiving unit DS1, a second light-receiving unit DS2 that receives a reflected light beam from the DVD information recording surface RL2, and a prism PS, and a diffractive optical element An objective lens in which both surfaces of the DOE and the laser beam transmitted through the diffractive optical element DOE have a function of condensing on the information recording surfaces RL1 and RL2. OBJ objective lens unit, biaxial actuator AC, diaphragm STO corresponding to the first numerical aperture NA1 = 0.85 of the high density optical disk HD, collimating lens COL, beam shaping lens SH, optical disk It comprises an aperture limiting element AP for limiting the numerical aperture of the objective lens unit when recording / reproducing information with respect to a DVD.

  In the optical pickup device PU1, when information is recorded / reproduced with respect to the high density optical disk HD, the first light emitting unit EP1 is caused to emit light. The divergent light beam emitted from the first light emitting unit EP1 is reflected by the prism PS as illustrated by the solid line in FIG. After being shaped and converted into a collimated light beam by the collimating lens COL, the diameter of the light beam is regulated by the stop STO, and then transmitted through the aperture limiting element AP. One spot is formed on the information recording surface RL1. The objective lens unit performs focusing and tracking by a biaxial actuator AC arranged around the objective lens unit. The reflected light beam modulated by the information pits on the first information recording surface RL1 is again transmitted through the objective lens unit, the aperture limiting element AP, the stop STO, the collimating lens COL, and the beam shaping lens SH, and reflected twice inside the prism PS. The light is condensed on the light receiving part DS1. Then, information recorded on the high density optical disk HD can be read using the output signal of the light receiving unit DS1.

  Further, in the optical pickup device PU1, when recording / reproducing information with respect to the optical disc DVD, the second light emitting unit EP2 is caused to emit light. The divergent light beam emitted from the second light emitting unit EP2 is reflected by the prism PS as shown by the broken line in FIG. After being shaped, converted into a parallel light beam by the collimating lens COL, and the light beam diameter is regulated by the aperture limiting element AP, it is formed on the second information recording surface RL2 via the second protective layer PL2 of the DVD by the objective lens unit. It becomes a spot. The objective lens unit performs focusing and tracking by a biaxial actuator AC arranged around the objective lens unit. The light again passes through the objective lens unit, the aperture limiting element AP, the stop STO, the collimating lens COL, and the beam shaping lens SH, is reflected twice inside the prism PS, and is condensed on the light receiving unit DS2. And the information recorded on DVD can be read using the output signal of light-receiving part DS2.

  Next, the configuration of the objective lens unit will be described. The diffractive optical element DOE and the objective lens OBJ shown in FIG. 1 are both plastic lenses, and the objective lens OBJ is an objective lens dedicated to the high-density optical disc HD. Further, around each optical region (region of the diffractive optical element DOE and objective lens OBJ through which the laser beam of wavelength λ1 passes), there are flange portions FL1 and FL2 formed integrally with the optical function portion. The flange portions FL1 and FL2 are integrated by joining a part thereof. Furthermore, the objective lens unit and the aperture limiting element AP are integrated via a lens frame B.

  In the region corresponding to the second numerical aperture NA2 of the optical surface S1 on the light source side of the diffractive optical element DOE, as schematically shown in FIG. 8, there are a plurality of annular zones in which a staircase structure is formed. A superposition type diffractive structure having a structure arranged around the optical axis is formed, and spherical aberration caused by the difference in thickness between the first protective layer PL1 and the second protective layer PL2 due to the diffractive action of the superposition type diffractive structure. By correcting the above, compatibility between the high-density optical disk HD and the optical disk DVD is achieved. Note that the principle of spherical aberration correction by the superposition type diffractive structure will be described in Example 3 to be described later, so a detailed description is omitted here.

  Further, on the optical surface S2 on the optical disk side of the diffractive optical element DOE, as schematically shown in FIG. 8, the planar annular zone is separated from the optical axis by a step d with respect to the annular zone adjacent to the inside. A diffractive structure having a configuration shifted to the optical disc side is formed.

This diffractive structure is a structure for suppressing the longitudinal chromatic aberration of the objective lens OBJ in a short wavelength region such as blue-violet and the spherical aberration change accompanying the change in incident wavelength, and the region corresponding to the second numerical aperture NA2. Then, the step d _IN is m = Int (| d _IN | · (N _λ1 −1) / λ ₁ ) = 2 (14)
In a region corresponding to the outside from the second numerical aperture NA2, the step d _OUT is n = Int (| d _OUT | · (N _λ1 −1) / λ ₁ ) = 4 ( 15)
The depth is set to satisfy. Here, N _.lambda.1 is the refractive index of the diffractive optical element DOE for the first wavelength lambda _1.

  In a region corresponding to the second numerical aperture NA2, when a light beam having the first wavelength λ1 is incident, + 2nd order diffracted light is generated with a diffraction efficiency of 100%, and a light beam having the second wavelength λ2 is incident. + 1st order diffracted light is generated with a diffraction efficiency of 87%. Thus, high diffraction efficiency is ensured for any wavelength in the region corresponding to the second numerical aperture NA2 through which the light beams of both wavelengths pass.

  In the diffractive optical element DOE of the present embodiment, a superposition type diffractive structure is formed on the optical surface S1 on the light source side and a diffractive structure is formed on the optical surface S2 on the optical disc side. A configuration may be adopted in which a diffractive structure is formed on the optical surface S1 on the light source side and a superposition type diffractive structure is formed on the optical surface S2 on the optical disc side.

  Further, in the diffractive optical element DOE of the present embodiment, a configuration using a superposition type diffractive structure as a structure for correcting spherical aberration caused by the difference in thickness between the first protective layer PL1 and the second protective layer PL2 However, a configuration using a diffractive structure as in Example 1 or Example 2 described later may be used.

<Second Embodiment>
FIG. 2 is a diagram schematically showing a configuration of the second optical pickup device PU2 capable of appropriately recording / reproducing information with respect to the high density optical disc HD. The optical pickup device PU2 and a support means (not shown) for supporting the optical disk constitute an optical information recording / reproducing device.

  The optical specifications of the high-density optical disk HD are the first wavelength λ1 = 405 nm, the thickness t1 of the first protective layer PL1 = 0.1 mm, and the first numerical aperture NA1 = 0.85. However, the combination of the wavelength, the thickness of the protective layer, and the numerical aperture is not limited to this.

  The optical pickup device PU2 is a blue-violet semiconductor laser LD that emits a 405 nm laser beam when recording / reproducing information on the high-density optical disk HD, and a reflection from the information recording surface RL1 of the high-density optical disk HD. Photodetector PD for receiving the light beam, diffractive optical element DOE, and objective lens having aspherical surfaces on both sides having a function of condensing the laser light beam transmitted through the diffractive optical element DOE on the information recording surfaces RL1 and RL2. OBJ, biaxial actuator AC, stop STO corresponding to the first numerical aperture NA1 = 0.85 of the high-density optical disc HD, collimating lens COL, beam shaping lens SH, polarizing beam splitter BS, and sensor lens SEN It is composed of. In place of the blue-violet semiconductor laser, a blue-violet SHG laser using second harmonic generation (SHG) may be used.

  In the optical pickup device PU2, when recording / reproducing information with respect to the high density optical disk HD, the blue-violet semiconductor laser LD is caused to emit light. The divergent light beam emitted from the blue-violet semiconductor laser LD is shaped from an elliptical shape to a circular shape by the beam shaping lens SH as shown by the solid line in FIG. 2, and is transmitted through the polarization beam splitter BS. After that, the light beam is converted into a parallel light beam by the collimating lens COL, transmitted through the diffractive optical element DOE, and the diameter of the light beam is regulated by the stop STO. Then, the objective lens OBJ passes through the first protective layer PL1 of the high-density optical disk HD. One spot is formed on the information recording surface RL1. The objective lens OBJ performs focusing and tracking by a biaxial actuator AC arranged around the objective lens OBJ. The reflected light beam modulated by the information pits on the first information recording surface RL1 is transmitted again through the objective lens OBJ, the stop STO, the diffractive optical element DOE, and the collimating lens COL, and is reflected by the polarization beam splitter BS, and then the sensor lens SEN. Astigmatism is given by passing through the light, and the light is condensed on the light receiving surface of the photodetector PD. Then, information recorded on the high density optical disk HD can be read using the output signal of the photodetector PD.

  On the optical surface S1 on the light source side of the diffractive optical element DOE, as schematically shown in FIG. 10, a diffractive structure having a sawtooth shape in cross section is formed on a plane. The shape of the annular zone is shifted to the light source side as it moves away from the optical axis by a level difference d with respect to the annular zone adjacent to the inside.

This diffractive structure is a structure for suppressing axial chromatic aberration of the objective lens OBJ in the blue-violet region. In the region corresponding to the entrance pupil diameter of the objective lens OBJ, the step d _IN is m = Int (| d _IN | • (N _λ1 −1) / λ ₁ ) = 2 (16)
In the region corresponding to the outside from the entrance pupil diameter of the objective lens OBJ, the step d _OUT is n = Int (| d _OUT | · (N _λ1 −1) / λ ₁ ) = 4 (17)
The depth is set to satisfy. Here, N _.lambda.1 is the refractive index of the diffractive optical element DOE for the first wavelength lambda _1.

  In the diffractive optical element DOE of the present embodiment, the refractive power on the paraxial axis of the optical surface S2 on the optical disc HD side is determined so that the absolute value thereof is equal to the absolute value of the diffractive power on the paraxial axis of the diffractive structure. Therefore, it can be arranged in a parallel light beam between the collimating lens COL and the objective lens OBJ.

  Further, in the present embodiment, the objective lens OBJ is a lens having a one-group configuration, but an objective lens including two or more lens groups may be used.

(Example)
Next, examples of the diffractive optical element that can be used in the above-described embodiment will be described. In addition, when the optical surface of the diffractive optical element in the following embodiments is configured as an aspherical surface, each aspherical surface has an aspherical shape represented by the following formula 1. Where Z (mm) is the amount of deformation from the plane contacting the vertex of the optical surface, h (mm) is the height in the direction perpendicular to the optical axis, r (mmn) is the paraxial radius of curvature, and κ is the cone coefficient , A _2i is an aspheric coefficient.

The diffractive structure formed in the diffractive optical element is represented by an optical path difference added to the transmitted wavefront by this diffractive structure. The optical path difference is expressed as follows. The height in the direction perpendicular to the optical axis is h (mm), B _2j is the optical path difference function coefficient, the wavelength of the incident light beam is λ (nm), the manufacturing wavelength is λ _B (nm), and the wavelength λ is Of the diffracted light generated by this diffractive structure when a light beam is incident, when the diffraction order of the diffracted light having the maximum diffraction efficiency is defined as do, the optical path difference function Φ _b (mm) defined by the following expression 2 It is represented by

  Example 1 is an objective lens unit that can be applied to the embodiment of FIG. 1 and is compatible with a high-density optical disk HD and a DVD that are composed of a diffractive optical element and an objective lens. The diffractive optical element is made of resin, and the objective lens is made of glass. The objective lens is an objective lens dedicated to the high density optical disc HD. The lens data according to Example 1 is shown in Table 1, the diffraction structure data is shown in Table 2, and the lens cross-sectional view is shown in FIG.

In Table 1, a power of 10 (for example, 2.5 × 10 ⁻⁰³ ) is represented using E (for example, 2.5E-03), and the same applies to the lens data tables thereafter.

In Table 1, NA ₁ is the numerical aperture of the high-density optical disk HD, NA ₂ is the numerical aperture of the DVD, f ₁ (mm) is the focal length with respect to the wavelength λ ₁ , and f ₂ (mm) is the focal length with respect to the wavelength λ ₂ . , Λ ₁ (nm) is the wavelength used for the high-density optical disk HD, λ ₂ (nm) is the wavelength used for the DVD, M ₁ is the magnification with respect to the wavelength λ ₁ , M ₂ is the magnification with respect to the wavelength λ ₂ , and t ₁ (mm) is protective layer thickness of the high density optical disk _HD, t 2 (mm) protective layer thickness of DVD, r (mm) is a paraxial radius of curvature, d1 (mm) is the axial distance relative to the wavelength λ _1, d2 (mm) is the wavelength lambda surface separation for _2, N _.lambda.1 is the refractive index for the wavelength lambda _1, N _.lambda.2 is the refractive index for the wavelength lambda _2, [nu] d is Abbe's number, dor is in the diffracted light generated when the light flux is incident of the diffractive structure, most diffraction Represents the diffraction order of diffracted light with high efficiency The same applies to the subsequent lens data.

In Table 2, the step depth sign is “−” when the diffraction zone is shifted to the light source side with respect to the diffraction zone adjacent to the inner side, and the diffraction zone is set to the diffraction zone adjacent to the inner side. On the other hand, the case of shifting to the optical disc side is set to “+”, and the same applies to the data of the diffraction structure thereafter.

  As shown in Table 2, the first surface of the diffractive optical element is formed with a diffractive structure composed of a total of 89 diffractive ring zones. As schematically shown in FIG. The diffraction ring zone is shifted by a step d with respect to the diffraction ring zone adjacent to the inside. Up to the 32nd ring zone is shifted to the light source side as it is away from the optical axis, and the outside from the 33rd ring zone is shifted to the optical disk side as it is away from the optical axis.

  Also, the diffraction zone inside from the 35th zone is shifted by 1.54 μm with respect to the diffraction zone adjacent to the inside, and the diffraction zone outside from the 36th zone is relative to the diffraction zone adjacent to the inside. This is shifted by 3.09 μm. That is, up to the 35th zone, m = 2 in the formula (1) or (7), and n = 4 in the formula (2) or (8) from the 36th zone.

This diffractive structure is a structure for correcting spherical aberration due to the difference in the protective layer thickness between the high-density optical disk HD and the DVD and achieving compatibility between the high-density optical disk HD and the DVD, and satisfies the equation (4). In this diffractive structure, 2-order diffracted light of the light flux of the wavelength lambda _1, most diffraction efficiency 1-order diffracted light of the light flux of wavelength lambda ₂ is increased, the diffraction efficiency in the interior from the 35 annular, the wavelength lambda ₁ of the light beam The second-order diffracted light is 100%, and the first-order diffracted light of the light beam having the wavelength λ ₂ is 88.2%.

  Example 2 is an objective lens unit that can be applied to the embodiment of FIG. 1 and is compatible with a high-density optical disk HD and DVD composed of a diffractive optical element and an objective lens. The diffractive optical element is made of resin, and the objective lens is made of glass. The objective lens is an objective lens dedicated to the high density optical disc HD. The lens data is shown in Table 3, the diffraction structure data is shown in Table 2, and the lens cross-sectional view is shown in FIG.

  As shown in Table 4, the second surface of the diffractive optical element is formed with a diffractive structure composed of a total of 65 ring zones. As schematically shown in FIG. The annular zone is shifted by a level difference d with respect to the diffraction zone adjacent to the inside. Up to the 23rd annular zone is shifted to the light source side as it is away from the optical axis, and the outside from the 24th annular zone is shifted to the optical disc side as it is away from the optical axis.

  In addition, the inner diffraction zone from the 30th annular zone is shifted by 2.32 μm relative to the diffraction zone adjacent to the inner side, and the outer diffraction zone from the 30th annular zone is shifted relative to the diffraction zone adjacent to the inner side. This is shifted by 3.86 μm. That is, up to the 30th annular zone, m = 3 in the formula (1) or (7), and n = 5 in the formula (2) or (8) outside the 30th annular zone.

This diffractive structure is a structure for correcting spherical aberration due to the difference in the protective layer thickness between the high-density optical disk HD and the DVD and achieving compatibility between the high-density optical disk HD and the DVD, and satisfies the equation (5). In this diffractive structure, the third-order diffracted light of the light beam of wavelength λ ₁ and the second-order diffracted light of the light beam of wavelength λ ₂ have the highest diffraction efficiency, and the diffraction efficiency inside the 30th annular zone is the light beam of wavelength λ ₁ . The third-order diffracted light is 100%, and the second-order diffracted light of the light beam having the wavelength λ ₂ is 86.4%.

  Example 3 is an objective lens unit that is compatible with a high-density optical disk HD and a DVD that are composed of a diffractive optical element and an objective lens that can be applied to the embodiment of FIG. Both the diffractive optical element and the objective lens are made of resin. The objective lens is an objective lens dedicated to the high density optical disc HD. The lens data is shown in Table 5, the diffraction structure data is shown in Table 6, and the lens cross-sectional view is shown in FIG.

  As shown in Table 6, the second surface of the diffractive optical element is formed with a diffractive structure composed of a total of 61 diffractive ring zones. As schematically shown in FIG. The diffraction ring zone is shifted to the optical disc side as the distance from the optical axis is increased by a level difference d with respect to the diffraction ring zone adjacent to the inside.

  Also, the diffraction zone inside from the 36th zone is shifted by 1.56 μm with respect to the diffraction zone adjacent inside, and the diffraction zone outside from the 37th zone is relative to the diffraction zone adjacent inside. Shifted by 3.11 μm. That is, up to the 36th ring zone, m = 2 in the expression (1) or (7), and n = 4 in the expression (2) or (8) outside the 37th ring zone.

This diffractive structure is a structure for correcting axial chromatic aberration and chromatic spherical aberration in the blue-violet wavelength region, and satisfies the equation (6). In this diffractive structure, the second-order diffracted light of the light beam with wavelength λ _{1 and} the _first-order diffracted light of the light beam with wavelength λ ₂ have the highest diffraction efficiency, and the diffraction efficiency inside the 36th annular zone is the light beam with wavelength λ ₁ . The third-order diffracted light is 100%, and the second-order diffracted light of the light beam having the wavelength λ ₂ is 88.2%.

The diffractive structure formed on the first surface of the diffractive optical element is a structure for correcting spherical aberration due to the difference in the protective layer thickness between the high-density optical disk HD and DVD and achieving compatibility between the high-density optical disk HD and DVD. It is. As schematically shown in FIG. 8, this diffractive structure has a structure in which a plurality of annular zones each having a staircase structure formed therein are arranged with the optical axis as the center (in this specification, such a diffraction structure). The structure is called “superimposed diffraction structure”). In this superposition type diffractive structure, the depth D per step of the staircase structure formed in each annular zone is:
D = 2 · λ ₁ / (N _λ1 −1) (μm) (18)
And the number of divisions of each annular zone is set to 5. However, λ ₁ represents, for example, the wavelength of a light beam emitted from a blue-violet semiconductor laser in units of micron (here, λ ₁ = 0.408 μm), and N _λ1 is a refraction with respect to the wavelength λ ₁ of the diffractive optical element. Rate (here, N _λ1 = 1.5242).

When a light beam having a wavelength λ ₁ is incident on the superposition type diffractive structure, an optical path difference of 2 × λ ₁ (μm) is generated between adjacent steps, and the light beam having a wavelength λ ₁ has a substantial phase difference. Since it is not given, it passes through without being diffracted. In the following description, a light beam that is transmitted without being substantially given a phase difference by the superposition type diffractive structure is referred to as zero-order diffracted light.

On the other hand, for example, when a light beam having a wavelength λ ₂ (here, λ ₂ = 0.658 m) emitted from a red semiconductor laser is incident on this superposition type diffractive structure, D × (N _λ2 between adjacent stairs. -1) An optical path difference of -λ ₂ = 0.13 μm is generated, and for one ring zone divided into five, the optical path for approximately one wavelength of 0.13 × 5 = 0.65 μm and wavelength λ ₂ Since a difference occurs, the wavefronts that have passed through the adjacent annular zones overlap each other with a shift of one wavelength. That is, the light beam having the wavelength λ ₂ becomes diffracted light diffracted in the first-order direction by the superposition type diffractive structure. In the objective lens unit of the present embodiment, the spherical aberration due to the difference in the protective layer thickness between the high-density optical disk HD and the DVD is corrected using the diffractive action with respect to the wavelength λ ₂ of the superposition type diffractive structure. N _λ2 is a refractive index with respect to the wavelength λ ₂ of the diffractive optical element (here, N _λ2 = 1.5064). At this time, the diffraction efficiency of the first-order diffracted light of the light beam having the wavelength λ ₂ is 87.3%, but the amount of light is sufficient for recording / reproducing information with respect to the DVD.

  Example 4 is an optical system dedicated to a high-density optical disc HD, which is configured by a diffractive optical element and an objective lens, and can be applied to the embodiment of FIG. Both the diffractive optical element and the objective lens are made of resin. The lens data is shown in Table 7, the diffraction structure data is shown in Table 8, and the lens sectional view is shown in FIG.

  As shown in Table 8, the first surface of the diffractive optical element is formed with a diffractive structure composed of a total of 56 ring zones, and the cross-sectional shape is as shown schematically in FIG. The diffraction ring zone having a sawtooth shape is shifted to the light source side as the distance from the optical axis is increased by a level difference d with respect to the diffraction ring zone adjacent to the inside.

  Also, the diffraction zone inside from the 44th zone is shifted by 1.55 μm with respect to the diffraction zone adjacent inside, and the diffraction zone outside from the 45th zone is relative to the diffraction zone adjacent inside. Shifted by 3.11 μm. That is, up to the 44th ring zone, m = 2 in the expression (1) or (7), and n = 4 in the expression (2) or (8) outside the 45th ring zone.

  This diffractive structure is a structure for correcting axial chromatic aberration in the blue-violet wavelength region of the objective lens, and satisfies the equation (6).

  As described above, the present invention has been described by the embodiments and examples. However, the present invention is not limited to these, and various modifications can be made within the scope of the technical idea of the present invention.

FIG. 1A shows a first light that can appropriately record / reproduce information on both the high-density optical disk HD that is the first optical information recording medium and the optical disk DVD that is the second optical information recording medium. It is a figure which shows schematically the structure of pick-up apparatus PU1. FIG. 1B is a front view of a light source used in the optical pickup device of FIG. It is a figure which shows schematically the structure of 2nd optical pick-up apparatus PU2 which can record / reproduce information appropriately with respect to the high-density optical disk HD. 3 is a cross-sectional view of an objective lens unit in Embodiment 1. FIG. 1 is a schematic cross-sectional view showing a diffractive structure of Example 1. FIG. 6 is a cross-sectional view of an objective lens unit in Embodiment 2. FIG. 6 is a schematic cross-sectional view showing a diffractive structure of Example 2. FIG. 10 is a cross-sectional view of an objective lens unit in Embodiment 3. FIG. 6 is a schematic cross-sectional view showing a diffractive structure of Example 3. FIG. 6 is a cross-sectional view of an optical system in Example 4. FIG. 6 is a schematic cross-sectional view showing the diffractive structure of Example 4. FIG. It is a schematic sectional drawing which shows the relationship between the width | variety of the diffraction ring zone of a diffraction structure, and the depth of a level | step difference.

Explanation of symbols

AC biaxial actuator AP Aperture limiting element B Mirror frame BS Polarizing beam splitter COL Collimating lens DOE Diffractive optical element DS1 Light receiving part DS2 Light receiving part DVD Optical disk EP1 Light emitting part EP2 Light emitting part FL1 Flange part HD High density optical disk LD For blue-violet semiconductor laser LM Laser module OBJ Objective lens PD Photodetector PL1 Protective layer PL2 Protective layer PS Prism PU1 Optical pickup device PU2 Optical pickup device RL1 Information recording surface RL2 Information recording surface S1 Optical surface S2 Optical surface SEN Sensor lens SH Beam shaping lens STO Aperture

Claims (21)

In a diffractive optical element having a diffractive surface formed with a diffractive structure composed of a plurality of annular zone steps,
Depth d _IN in the optical axis direction of the annular zone step inside the predetermined height from the optical axis in the optical surface, and a depth d _{OUT in} the optical axis direction of the annular zone step outside the predetermined height, A diffractive optical element that satisfies the following expressions (1) to (3), respectively.
m = Int (| d _IN | · (N−1) / λ) (1)
n = Int (| d _OUT | · (N−1) / λ) (2)
m <n (3)
Where Int (X) is an integer closest to X, λ is the wavelength of the light beam incident on the diffractive structure, and N is the refractive index of the diffractive optical element at the wavelength λ.   The diffractive optical element according to claim 1, wherein the wavelength λ is 450 nm or less. The light beam having a wavelength longer than the wavelength λ is greater than the predetermined height with respect to the diffraction order at which the diffraction efficiency becomes maximum when the light beam having the wavelength λ is incident on the annular zone step inside the predetermined height. 3. The diffractive optical element according to claim 1, wherein the d _IN is determined so that the diffraction order that maximizes the diffraction efficiency when incident on the inner annular zone step is small.   The diffractive optical element according to claim 3, wherein the wavelength λ is 450 nm or less, and a wavelength longer than the wavelength λ is in a range of 640 nm to 690 nm. The wavelength λ is λ ₁ , the wavelength longer than the wavelength λ is λ ₂ , the refractive index of the diffractive optical element at the wavelength λ ₁ is N _λ1 , and the refractive index of the diffractive optical element at the wavelength λ ₂ is N _λ2. When the light beam having the wavelength λ ₁ is incident on the annular zone step inside the predetermined height, the diffraction order that maximizes the diffraction efficiency is m ₁ , and the light beam having the wavelength λ ₂ is from the predetermined height. When the diffraction order that maximizes the diffraction efficiency when incident on the inner annular zone step is m ₂ , the following equation (4) is satisfied, and the diffraction structure is used when the wavelength of the incident light beam becomes longer. 5. The diffractive optical element according to claim 4, wherein the spherical aberration has a wavelength dependency such that the spherical aberration changes in an overcorrection direction.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)}> 1 (4) The wavelength λ is λ ₁ , the wavelength longer than the wavelength λ is λ ₂ , the refractive index of the diffractive optical element at the wavelength λ ₁ is N _λ1 , and the refractive index of the diffractive optical element at the wavelength λ ₂ is N _λ2. When the light beam having the wavelength λ ₁ is incident on the annular zone step inside the predetermined height, the diffraction order that maximizes the diffraction efficiency is m ₁ , and the light beam having the wavelength λ ₂ is from the predetermined height. When the diffraction order that maximizes the diffraction efficiency when incident on the inner annular zone step is m ₂ , the following equation (5) is satisfied, and the diffractive structure is used when the wavelength of the incident light beam becomes longer. 5. The diffractive optical element according to claim 4, wherein the spherical aberration has a wavelength dependency such that the spherical aberration changes in a direction of insufficient correction.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)} <1 (5)   The diffractive optical element further includes an optical surface whose refractive power in the paraxial axis is negative, and the diffractive structure is formed on an optical surface whose refractive power in the paraxial axis is negative. The diffractive optical element according to claim 1, wherein the cross-sectional shape including the optical axis is a step shape.   The diffractive optical element further has an optical surface whose refractive power in the paraxial axis is negative, and the diffractive structure is formed on a macroscopic plane opposite to the optical surface where the refractive power in the paraxial axis is negative. The diffractive optical element according to claim 1, wherein a cross-sectional shape including an optical axis of the diffractive structure is a sawtooth shape. The diffraction power on the paraxial axis of the diffraction structure with respect to the wavelength λ is P ₁ (mm ⁻¹ ), the diffraction power on the paraxial axis of the diffraction structure with respect to a wavelength longer than the wavelength λ by 5 nm is P ₂ (mm ⁻¹ ), and the wavelength. 9. The following expression (6) is satisfied, where P ₃ (mm ⁻¹ ) is a paraxial power of the diffraction structure for a wavelength shorter than λ by 5 nm. The diffractive optical element according to 1.
P ₂ > P ₁ > P ₃ (6) An objective lens for an optical pickup, which is integrated with the diffractive optical element according to any one of claims 1 to 9. An optical pickup device having a first light source that emits a light beam having a _first wavelength λ ₁ and a condensing optical system including an objective lens, the optical pick-up device via the condensing optical system from the first light source In the optical pickup device capable of recording and / or reproducing information by condensing the light beam on the information recording surface of the first optical information recording medium,
The condensing optical system has a diffractive optical element having a diffractive surface on which a diffractive structure composed of a plurality of annular zone steps is formed,
In the diffractive optical element, the inside of the annular zone step from a predetermined height from the optical axis of the optical surface and the depth d _IN of the optical axis, the optical axis direction of the annular zone step outside than the predetermined height A depth d _OUT satisfies the following expressions (7) to (9), respectively.
m = Int (| d _IN | · (N _λ1 −1) / λ ₁ ) (7)
n = Int (| d _OUT | · (N _λ1 −1) / λ ₁ ) (8)
m <n (9)
Where Int (X) is an integer closest to X, λ ₁ is the wavelength of the light beam incident on the diffractive structure, and N _λ1 is the refractive index of the diffractive optical element at the wavelength λ _1. is there. The optical pickup device according to claim 11, wherein the first wavelength λ ₁ is 450 nm or less. Further, the optical pickup device includes a second light source that emits a light beam having a second wavelength λ ₂ (λ ₂ > λ ₁ ) longer than the first wavelength λ ₁ , and the optical pickup device passes through the condensing optical system. Thus, it is possible to record and / or reproduce information by condensing the light beam from the second light source on the information recording surface of the second optical information recording medium,
For the diffraction order at which the diffraction efficiency is maximized when the light beam of the first wavelength λ ₁ is incident on the annular zone step inside the predetermined height, the light beam of the second wavelength λ ₂ is The _dIN is determined so that a diffraction order that maximizes diffraction efficiency when incident on the annular zone step inside the predetermined height is reduced. Optical pickup device. 14. The optical pickup device according to claim 13, wherein the first wavelength [lambda] ₁ is 450 nm or less, and the second wavelength [lambda] ₂ is in the range of 640 nm to 690 nm. The refractive index of the diffractive optical element at the first wavelength λ ₁ is N _λ1 , the refractive index of the diffractive optical element at the second wavelength λ ₂ is N _λ2 , and the light beam of the first wavelength λ ₁ is The diffraction order at which the diffraction efficiency is maximized when entering the annular zone step inside the predetermined height is m ₁ , and the luminous flux having the _second wavelength λ ₂ is inside the predetermined height. When the diffraction order that maximizes the diffraction efficiency when incident on a step is m ₂ , the following equation (10) is satisfied, and the diffractive structure is overcorrected for spherical aberration when the wavelength of the incident light beam becomes long. The optical pickup device according to claim 14, wherein the optical pickup device has wavelength dependency of spherical aberration that changes in a direction.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)}> 1 (10) The refractive index of the diffractive optical element at the first wavelength λ ₁ is N _λ1 , the refractive index of the diffractive optical element at the second wavelength λ ₂ is N _λ2 , and the light beam of the first wavelength λ ₁ is The diffraction order at which the diffraction efficiency is maximized when entering the annular zone step inside the predetermined height is m ₁ , and the luminous flux having the _second wavelength λ ₂ is inside the predetermined height. When the diffraction order that maximizes the diffraction efficiency when incident on a step is m ₂ , the following equation (11) is satisfied, and the diffractive structure has insufficient correction of spherical aberration when the wavelength of the incident light beam becomes long The optical pickup device according to claim 14, wherein the optical pickup device has wavelength dependency of spherical aberration that changes in a direction.
{M ₁ · λ ₁ / (N _λ1 -1)} / {m ₂ · λ ₂ / (N _λ2 -1)}> 1 (11)   The diffractive optical element further includes an optical surface whose refractive power in the paraxial axis is negative, and the diffractive structure is formed on an optical surface whose refractive power in the paraxial axis is negative. The optical pickup device according to any one of claims 11 to 16, wherein the cross-sectional shape including the optical axis is a step shape.   The diffractive optical element further has an optical surface whose refractive power in the paraxial axis is negative, and the diffractive structure is formed on a macroscopic plane opposite to the optical surface where the refractive power in the paraxial axis is negative. The optical pickup device according to claim 11, wherein a cross-sectional shape including an optical axis of the diffractive structure is a sawtooth shape. The diffraction power at the paraxial axis of the diffractive structure with respect to the first wavelength λ ₁ is P ₁ (mm ⁻¹ ), and the diffraction power at the paraxial axis of the diffractive structure with respect to a wavelength 5 nm longer than the first wavelength λ ₁ is P _2. (Mm ⁻¹ ), where P ₃ (mm ⁻¹ ) is the paraxial power of the diffraction structure for a wavelength shorter than the first wavelength λ ₁ by 5 nm, and satisfies the following expression (12): The optical pickup device according to any one of claims 11 to 18.
P ₂ > P ₁ > P ₃ (12)   20. The optical pickup device according to claim 11, wherein the diffractive optical element is integrated with the objective lens. 21. The optical pickup device according to claim 11, and an optical information recording medium support unit that supports the optical information recording medium so that an information signal can be recorded and / or reproduced by the optical pickup device. An optical information recording / reproducing apparatus comprising:

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